The invention relates to a digital=to-analog converter (DAC), particularly for systems requiring an ultra fast high resolution conversion of a digital input code into a corresponding analog output current or voltage.
Conventional DACs comprise a switch matrix controlled by the digital input code, and an associated resistor network which is used for obtaining binary weighted partial currents or voltages for a further adding in a summing unit. The partial currents are obtained by current sources each coupled to the summing unit directly or via a separate switch. The summing unit may also comprise an additional resistor network. The accuracy of the current sources practically determines the accuracy of the DAC. Furthermore, the switch on-resistance, linearity, temperature coefficient, etc. are insignificant. An output amplifier is necessary for obtaining a voltage proportional to the input code.
The DACs employing the resistor network for obtaining binary weighted voltages use switches having precisely matched characteristics as the on-resistance thereof is added to selected resistances of the resistor network. A constant voltage drop across every closed switch must be maintained. However, the circuit structure is fairly simple as the reference voltage is directly divided by the resistor network.
Several designs combine good and bad features of both techniques described above. For instance, commonly known quad current source approach is based on binary weighted current sources, in groups of four. Parts of the circuit are repetitive which makes easier a monolithic integration. For instance, three such quads provide a 12 bit resolution. The circuit still requires current sources operating with three different currents and two accurately trimmed resistor networks with low temperature coefficients.
The disadvantages of conventional DACs are many. The binary weighted currents or voltages employ respectively precisely matched switches or transistors of the current sources. Specific serial and/or parallel connections of the actually resistive switches or the current sources are often necessary for matching respective parameters and their temperature coefficients. As a result the monotonicity is difficult to achieve and the long term stability is poor. The parasitic capacitances as well as switching delays of the analog switches cause high transient voltage spikes. The output impedance of the DACs is limited and often depends on the input code. This must be considered in an output amplifier.